1. Technical Field
The present disclosure relates to a nitride semiconductor light-emitting device including a semiconductor light-emitting chip which includes a nitride semiconductor active layer having a non-polar plane or a semi-polar plane as a growth plane.
2. Description of the Related Art
Nitride semiconductors containing nitrogen (N) in a group V element is considered promising as a material of a short wavelength light-emitting element because of a large bandgap thereof. Among the nitride semiconductors, gallium nitride-based compound semiconductors have been actively studied. Blue light emitting diode (LED) elements, green LED elements and blue semiconductor laser elements using a gallium nitride-based compound semiconductor have been put into practice.
A gallium nitride-based compound semiconductor contains a compound semiconductor having a part of gallium (Ga) substituted with at least one of aluminum (Al) and indium (In). Such a nitride semiconductor is represented by general formula AlxGayInzN (where 0≦x<1, 0≦z<1, 0<y≦1, x+y+z=1). Hereinafter, a gallium nitride-based compound semiconductor will be referred to as a GaN-based semiconductor.
In a GaN-based semiconductor, the bandgap can be greater than that of GaN by replacing Ga atoms with Al atoms. The bandgap can be smaller than that of GaN by replacing Ga atoms with In atoms. This allows, for example, blue or green short wavelength light and also, for example, orange or red long wavelength light to be emitted. Owing to such a feature, a nitride semiconductor light-emitting element is expected to be applied for an image display device and an illumination device.
A nitride semiconductor has a wurtzite crystalline structure. FIGS. 1A, 1B and 1C each show a plane orientation of a wurtzite crystalline structure with a 4-index notation (hexagonal index notation). According to the 4-index notation, crystalline surfaces and the plane orientations thereof are expressed by use of elementary vectors represented by a1, a2, a3 and c. The elementary vector c extends in a [0001] direction, and an axis in this direction is referred to as a “c-axis”. A plane perpendicular to the c-axis is referred to as a “c-plane” or “[0001] plane”. FIG. 1A shows the c-plane and also an a-plane, i.e., a (11-20) plane, and an m-plane, i.e., (1-100) plane. FIG. 1B shows an r-plane, i.e., a (1-102) plane. FIG. 1C shows a (11-22) plane. In this specification, symbol (−) provided to the left of the numeral in the parentheses that represents a Miller index indicates inversion of the index for the sake of convenience.
FIG. 2A shows a crystalline structure of a GaN-based semiconductor with a ball-and-stick model. FIG. 2B shows a ball-and-stick model of an atomic structure in the vicinity of the m-plane which is observed from the a-axis direction. The m-plane is perpendicular to the paper sheet of FIG. 2B. FIG. 2C shows a ball-and-stick model of an atomic structure on a surface of the +c-plane which is observed in the m-axis direction. The c-plane is perpendicular to the paper sheet of FIG. 2C. As can be seen from FIGS. 2A and 2B, N atoms and Ga atoms are located on a plane parallel to the m-plane. As can be seen from FIGS. 2A and 2C, on the c-plane, there are a layer in which only the Ga atoms are located and a layer in which only the N atoms are located.
Conventionally, for producing a semiconductor element by use of a GaN-based semiconductor, a substrate having the c-plane, namely, the (0001) plane as a main plane is used as a substrate on which a nitride semiconductor crystal is to be grown. In this case, the positions of the Ga atoms and the N atoms causes spontaneous electrical polarization to be formed in the c-axis direction in the nitride semiconductor. Therefore, the “c-plane” is also referred to as a “polar plane”. As a result of the electrical polarization, a piezo electric field is generated along the c-axis direction in a quantum well formed of InGaN, which forms a light emitting layer of a nitride semiconductor light-emitting element. Due to the generated piezo electric field, there occurs a positional shift in the distribution of electrons and halls in the light emitting layer. This causes a problem that the internal quantum efficiency of the light emitting layer is decreased due to the quantum-confined Stark effect of the carriers. In order to suppress the decrease in the internal quantum efficiency of the light emitting layer, the light emitting layer formed on the (0001) plane is designed to have a thickness of 3 nm or less.
Recently, it has been studied to produce a light-emitting element by use of a substrate having an m-plane or an a-plane referred to as a non-polar plane or a −r-plane or a (11-22) plane referred to as a semi-polar plane as a main plane. As shown in FIG. 1, in the wurtzite crystalline structure, the m-planes are six equivalent planes which are parallel to the t-axis and perpendicular to the c-plane. For example, in FIG. 1, the (1-100) plane perpendicular to the [1-100] direction is the m-plane. The other m-planes equivalent to the (1-100) plane include a (−1010) plane, a (10-10) plane, a (−1100) plane, a (01-10) plane and a (0-110) plane.
As shown in FIGS. 2A and 2B, on the m-plane, the Ga atoms and the N atoms are present on the same atomic plane. Therefore, electrical polarization is not caused in a direction perpendicular to the m-plane. For this reason, when a light-emitting element is produced by use of a semiconductor stacking structure having the m-plane as a growth plane, the piezo electric field is not generated in the light emitting layer. Thus, the problem that the internal quantum efficiency is decreased due to the quantum-confined Stark effect of the carriers is solved. This is also applicable to the a-plane, which is a non-polar plane other than the m-plane, and a similar effect is provided with the −r-plane or the (11-22) plane, which are referred to as a semi-polar plane.
A nitride semiconductor light-emitting element including an active layer having the m-plane, the a-plane, the −r-plane or the (11-22) plane as a growth plane has a polarization characteristic derived from the structure of a valence band thereof.
For example, Japanese Laid-Open Patent Publication No. 2008-109098 discloses a light emitting diode device including a light emitting diode chip which includes a light emitting layer having a main surface and a package having a chip-located surface on which the light emitting diode chip is to be located. The light emitting diode chip is provided for the purpose of decreasing an intensity difference of light going out of the package that is caused by the difference in the in-plane azimuth angle of the chip-located surface. The light going out of the main surface of the light emitting layer has a plurality of different levels of intensity in accordance with the in-plane azimuth angle of the main plane of the light emitting layer. At least one of the light emitting diode chip and the package has a structure of decreasing the intensity difference of the light going out of the package that is caused by the difference in the in-plane azimuth angle of the chip-located surface.
Japanese Laid-Open Patent Publication No. 2010-238846 discloses a light-emitting device including a cover member containing a light reflecting material, a light-transmissive member facing a light emitting-side surface and a surface of the cover member, a light-emitting element provided as a light source section having a part thereof embedded in the cover member, and a wavelength converting member excitable by the light-emitting element. This light-emitting device is provided for increasing the light extracting efficiency.
Japanese Laid-Open Patent Publication No. 2009-123803 discloses an LED device including an LED chip and a package for accommodating the LED chip. In the LED chip, anisotropy of the light emitting intensity is caused in accordance with the azimuth angle in the main plane of the light emitting layer. In an area having a prescribed range of angles encompassing an azimuth angle at which the light emitting intensity from the LED chip is high, a high resin-mold concentration part is located. In an area having a prescribed range of angles encompassing an azimuth angle at which the light emitting intensity from the LED chip is low, a low resin-mold concentration part is located.